Selenium is a trace element important for proper physiological function in humans. Selenium is ingested through the diet which can have a varying content of selenium.
Selenium is known to play a critical role in sustaining physiologic metabolism, growth, reproductive health, and immunity. Selenium is incorporated into different organic molecules including, for example, amino acids such as 1-selenomethionine, selenocysteine, and selenocystine. Thus, selenium can be a component part of proteins, many of which are of structural importance to the body. Furthermore, selenium is an important ingredient in a number of enzymes which influence metabolism, reproduction, the prevention of cancer, and immune defense in humans (See, e.g., Rayman, M, Lancet 356:233-241 (2000)).
Multiple studies have attempted to reveal potential health benefits resulting from the ingestion of low levels of selenium. For example, low concentrations of an inorganic form of selenium, have shown some potential health benefits (See, e.g., Furnsinn et al., Int. J. of Obesity and Related Metab. Dis., 19, 458-463 (1995)). However, at elevated dosage levels, beneficial effects are reversed and dangerous toxicity is manifested.
Research over the last two decades has suggested that selenium is effective in the reduction of cancer incidence when provided to animals at doses only 5- to 10-fold above nutritional requirement (See, e.g., El-Bayoumy, The role of selenium in cancer prevention, Philadelphia, Lippincott, 1-15, 1991). Chemoprevention studies with selenium in animal model systems have indicated that this element is effective for most, if not all of the organ systems and is protective against carcinogenic effects (See, e.g., El-Bayoumy, The role of selenium in cancer prevention, Philadelphia, Lippincott, 1-15, 1991. Both epidemiological studies and supplementation trials have also supported its efficacy in lowering the incidence of cancers of the liver, colon, prostate and lung (See, e.g., Yu et al. Biol Trace Elem Res, 56: 117-124 (1997); Clark et al., J Am Med Assoc, 276: 1957-1963 (1996); Yoshizawa et al., J Natl Cancer Inst, 90: 1219-1224, (1998); Brooks, et al., J Urol, 166: 2034-2038, (2001)). Other studies have demonstrated no beneficial effect for selenium reduction of cancers (See, e.g., Garland et al., J. Am. Coll Nutr., 12: 400-11 (1993); Ghadirian et al., Cancer Detect Prev, 24: 305-13 (2000)).
Multiple forms of selenium have been examined. These include inorganic selenium such as sodium selenite, and organic sources, including selenium yeast. There is a significant difference between toxicity of inorganic and organic selenium, the inorganic compounds usually being absorbed and utilized less efficiently and also being more toxic than organic sources of selenium.